Enantioselective intermolecular aldol catalysed by proline
Acetone and 2-methylpropanal

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The reaction goes via an enamine, which attacks the aldehyde Re-face to form the R-product. In the transition state the isopropyl group is pseudoequatorial to minimise the 1,3-diaxial repulsions from the axial methyl group alpha to the nitrogen. This forms the R-aldol product.

Click for more information on Re and Si.

B. List, R. A. Lerner and C. F. Barbas, J. Am. Chem. Soc., 2000, 122, 2395–2396.

J. Liu and L. Wang, Synthesis (Stuttg)., 2016, 49, 960–972.